The defining metric of what is competitive in the photovoltaics industry is the cost per kilowatt-hour. The success of a specific photovoltaic technology depends on its ability to reduce the manufacturing cost and increase the efficiency of the solar cells. In the integrated circuit (IC) industry, the cost of the substrate is frequently a negligible fraction of the total manufacturing cost. On the other hand, in the photovoltaics industry, the cost of the substrate material may ultimately determine the viability of the produce. The cost and the quality of substrates used for photovoltaic solar cells is critical in the manufacture of the cells.
More than 90% of photovoltaic solar cell manufacturers use silicon as a substrate material, and silicon wafers can account for as much as 45% of the total cost of photovoltaic modules. Hence, reducing the manufacturing cost of substrate materials without sacrificing quality is a potential opportunity for decreasing the cost of photovoltaics. Such an improvement, which is critical for both silicon and thin-film photovoltaics, should be achieved without a substantial reduction of the conversion efficiency of the photovoltaic cells.
Photovoltaic cells can be manufactured using electronic-grade silicon wafers; however, an increasingly common approach to reducing the cost of the photovoltaic substrate material is the use of lower quality (i.e., more impure) silicon sources and the replacement of single crystal silicon with multicrystalline silicon. Multicrystalline silicon can be fabricated by casting ingots and slicing the ingots into wafers. A growing number of manufacturers further reduce the cost of the substrate material by replacing sliced wafers with sheet or ribbon technology, which creates monocrystalline or multicrystalline silicon directly from a silicon melt. One such technology is the “String Ribbon Crystal Growth Technology” used by Evergreen Solar, Inc. Multicrystalline silicon suffers from a higher level of contamination due to the use of a less pure silicon source and also from a higher density of crystalline defects than single crystal (electronic-grade) silicon. Multicrystalline substrates are less expensive to manufacture, but photovoltaic cells made of multicrystalline silicon are also less efficient than cells made from single crystal silicon.
Transition metals are often a main culprit in degrading the efficiency of multicrystalline silicon solar cells. Multicrystalline photovoltaics also suffer from a large density of crystalline defects such as grain boundaries, as illustrated in FIG. 1. The density of grain boundaries, which is related to the size of crystallites in the sample, is one of the major parameters for differentiation between various silicon growth technologies.
The use of thin films of photovoltaic material deposited directly onto rigid or flexible substrates offers additional cost savings. Thin film cells use around one-one hundredth the amount of semiconductor material used in single crystal or multicrystalline cells. Furthermore, in addition to silicon, the deposited photovoltaic material can be other inorganic semiconductors or even organic materials with optical properties better suited to the solar spectrum. In silicon, for instance, the theoretical upper limit of efficiency (i.e., the conversion of light to electricity) under normal full-sun illumination is 31%. This limit is largely a result of the 1.1 eV band gap of silicon, which corresponds to a photon wavelength of 1127 nm. In practice, systems using crystalline silicon as a photovoltaic material are only about 20-25% efficient. In contrast, a more efficient photovoltaic cell is a three-junction indium gallium phosphide/indium gallium arsenide/ germanium cell, which increases the conversion efficiency to 37%. Fundamentally different devices, however, could have a theoretical efficiency as high as 87%. However, because non-silicon photovoltaic material is often based on compound semiconductors (e.g., copper indium gallium diselenide, or CIGS), the concentration of defects in such thin films can be even higher than the concentration in silicon.
Improving manufacturing processes and increasing manufacturing yields in photovoltaic fabrication requires reliable information regarding critical electrical properties of the substrate materials. One of the most critical parameters, the minority carrier lifetime τ, defines the quality of the photovoltaic substrate material and the efficient of the solar cells. The minority carrier lifetime, a measure of how long carriers (i.e., electrons and holes) remain before recombining, is highly sensitive to the presence of defects detrimental to device performance. A related parameter, the minority carrier diffusion length, is the average distance a carrier moves from its point of generation before it recombines. The longer the lifetime, the longer the diffusion length, and the higher the probability that optically generated carriers will reach the collection point (i.e., the p-n junction) and produce current in an external circuit. A shorter lifetime translates to a lower conversion efficiency of the solar cell and a poorer economic viability of the product.
The dominant approaches to measurement of the carrier lifetime in the photovoltaic industry are photoconductive decay (PCD) methods. In this technique, electron-hole pairs are created by an optical excitation that changes the conductivity of the sample. The recombination of excess carriers is monitored by measuring the decay of the conductivity after the illumination is turned off. Optical excitation with a pulse having a photon energy that exceeds the energy gap of the semiconductor generates excess electron-hole pairs, increasing the conductivity of the semiconductor (i.e., photoconductance). Relaxation of this photoconductance to the initial equilibrium state is detected using either radio frequency techniques (RF-PCD) or microwave reflectance techniques (μ-PCD). Both RF-PCD and μ-PCD measure an ‘effective total lifetime.’ In this approach, separation of the bulk lifetime from the surface recombination requires surface passivation, such as immersion of silicon in HF, iodine passivation, or corona charging. Alternatively, the requirement for surface passivation can be reduced by measuring slabs of material at least ten times thicker than standard photovoltaic substrates and using deeply penetrating illumination (i.e., longer wavelength). In μ-PCD, a microwave beam is directed to the sample and the reflected microwave power, which is proportional to the conductance of the sample, is measured. μ-PCD systems, unlike RF-PCD systems, exhibit a non-linear dependence on the injection level, and thus meaningful μ-PCD measurements are limited to a narrow range of excess carrier concentrations.
Another approach to measuring the minority carrier lifetime is the surface photovoltage (SPV) method. SPV measures the minority carrier diffusion length, which is directly related to the minority carrier lifetime and which can be used interchangeably with the lifetime in material characterization. An advantage of SPV over PCD is the ability of SPV to distinguish between bulk and surface carrier recombination. However, SPV requires a sample thickness larger than the carrier diffusion length which, for typical materials used in commercial solar cells (carrier lifetime ˜2-5 μs), exceeds the standard thickness of silicon photovoltaic wafers (around 100-200 μm). Additionally, in the SPV method, uncontrolled surface recombination at the wafer backside may strongly interfere with carrier recombination in the bulk, leading to misleading data. Furthermore, extended imperfections such as grain boundaries and dislocations can also interfere with SPV measurements, reducing their reliability.
Differentiation between recombination processes occurring in the bulk of the device and at the interfaces can be accomplished by identifying the dependence of lifetime on the geometry of the device being tested or on the excitation wavelength. Determining the underlying physics of the recombination process can be accomplished by analyzing the dependence of the lifetime on the level of photo-excitation and temperature. Such an approach, while cumbersome and time consuming, is quite effective in the R&D environment; however, it is not suitable for in-line monitoring of the manufacturing process.
The quasi-steady-state photoconductance (QSSPC) method measures the dependence of the lifetime (multiplied by the carrier mobility) on the level of photo-excitation from a quasi steady-state value of the photoconductance. In QSSPC, as in the PCD method, separation of the bulk lifetime from surface recombination requires surface passivation unless performed on slabs of material at least ten times thicker than standard photovoltaic substrates.
Separation of the effects of surface or extended crystallographic imperfections from the effect of bulk contaminants and bulk point defects is critical for process optimization and control of the fabrication of photovoltaics. The ability to maintain a low concentration of electrically active metallic impurities and crystalline defects is essential for achieving a high yield in silicon manufacturing and thus for reducing the cost of photovoltaic devices.